Block-type plate heat exchanger with anti-fouling properties

ABSTRACT

An block-type plate that has a stack of heat transfer plates which includes a first heat transfer plate and a second heat transfer plate. At least a part of each of the first heat transfer plate and the second heat transfer plate comprises a coating that i) has a layer thickness of 1-30 μm, ii) is prepared by sol-gel processing, iii) comprises silicon oxide (SiOx) having an atomic ratio of O/Si&gt;1, and iv) comprises ≧10 atomic percent carbon (C).

TECHNICAL FIELD

The invention relates to a block-type plate heat exchanger thatcomprises a top head, a bottom head and four side panels that are boltedtogether with a set of corner girders to form a sealed enclosure. Astack of heat transfer plates is arranged in the sealed enclosure. Theblock-type plate heat exchanger has properties that reduce fouling andfacilitate cleaning the heat exchanger.

BACKGROUND ART

Today several different types of plate heat exchangers exist and areemployed in various applications depending on their type. One certaintype of plate heat exchanger is assembled by bolting a top head, abottom head and four side panels to a set of corner girders to form abox-like enclosure around a stack of heat transfer plates. This certaintype of plate heat exchanger is referred to as a block-type heatexchanger. One example of a commercially available block-type heatexchanger is the heat exchanger offered by Alfa Laval AB under theproduct name Compabloc. Other block-type plate heat exchangers aredisclosed in patent documents EP165179 and EP639258.

In the block-type plate heat exchanger fluid paths for two heat exchangefluids are formed between the heat transfer plates in the stack of heattransfer plates. During operation fouling of the heat transfer plates isof concern, for example due to deposits, microbial growth, dirt etc.that arise from the fluids that pass between the heat transfer plates.Fouling typically reduces a heat transfer capability and increases apressure drop of the heat exchanger, which lead to an overall reducedperformance. The problem of fouling is typically solved by removing oneor more of the side panels such that the stack of heat transfer platesmay be accessed and the plates may be cleaned.

For other types of heat exchangers it is known to coat areas of the heatexchanger that are susceptible to fouling. Examples of coatingtechniques may be found in a number of patent documents, such as inUS20090123730, US20060196644, WO2008119751 and WO2009034359.

Even though the these coating techniques may reduce fouling, it appearsthat they are not optimal for a block-type plate heat exchanger thattypically is used in aggressive, high pressure applications where safetydemands are high. For example, the coating would typically after sometime be worn of its coating surface. Moreover, the unique design andstructure of the block-type plate heat exchanger calls for a differentcoating that has been optimized in respect of the inherent designstructure of the block-type plate heat exchanger.

SUMMARY

It is an object of the invention to find a coating that reduces foulingof a block-type plate heat exchanger. Another object is to findembodiments of a block-type plate heat exchanger that ensure that thecoating stays on the coated areas for a long operational time of theheat exchanger.

To fulfill these objects a block-type plate heat exchanger is provided.The block-type heat exchanger comprises a top head, a bottom head andfour side panels that are bolted together with a set of corner girdersto form a sealed enclosure, and a stack of heat transfer plates that isarranged in the sealed enclosure. The stack of heat transfer platescomprises pairs of heat transfer plates that are stacked such that aflow path for a first fluid is formed between the stacked pairs of heattransfer plates, wherein a pair of the stacked pairs of heat transferplates comprises a first heat transfer plate and a second heat transferplate that are joined such that a flow path for a second fluid is formedbetween the first and second heat transfer plates. At least a part ofeach of the first heat transfer plate and the second heat transfer platecomprises a coating that i) has a layer thickness of 1-30 μm, ii) isprepared by sol-gel processing, iii) comprises silicon oxide (SiOx)having an atomic ratio of O/Si>1, and iv) comprises ≧5 or ≧10 atomicpercent carbon (C).

The block-type plate heat exchanger is advantageous in that fouling ofthe heat transfer plates is significantly reduced. As a consequence noor less cleaning is required. This reduces use of strong detergentsand/or potentially abrasive, mechanical cleaning as well as reduces anoperational downtime of the plate heat exchanger. Moreover, the coatingis, comparison with prior art coatings, quite wear resistant and has arelatively resistance against formation of cracks in the coating whichotherwise might from due to torque and tension forces that act on theheat transfer plates. Generally, each side or each both sides of therespective heat transfer plate may comprise the coating.

The plate heat exchanger may have predetermined measurements for anumber of the components it comprises. For example, the first heattransfer plate and the second heat transfer plate may have a thicknessof 0.6-1.4 mm or 0.8-1.2 mm. Each of the first heat transfer plate andthe second heat transfer plate may have a heat transfer area of0.05-0.30 m² or 0.6-1.8 m². Any of the top head and the bottom head mayhave a thickness of 45-145 mm or 190-250 mm. Each of the four sidepanels may have a thickness of 35-85 mm or 65-175 mm. Each of the cornergirders may comprise a cross-sectional side that measures 35-85 mm or110-190 mm. Finally, the sealed enclosure may have a volume of 0.02-0.40m³ or 0.7-5.0 m³.

Empirical tests as well as finite element-based analysis have shown thateach of these measurements, either alone or one or more in combination,provide a structure of the heat exchanger that is particularly suitablefor the coating. The underlying reasons for this is that themeasurements provide a structure for the heat transfer plates thatprevents extensive flexing of the heat transfer plates when the heatexchanger is operated. This is of great advantage since the coating thenremains on the plates for a long period of time (flexing cause thecoating to fall off or wear out faster). Thus, the coating together withone or more of the predetermined measurements provide a block-type heatexchanger that has been optimized in respect of resisting fouling for alonger period of time.

The layer thickness of the coating may be 1.5-25 μm, or 2-20 μm, or 2-15μm, or 2-10 μm, or 3-10 μm. The silicon oxide, SiOx, may have an atomicratio of O/Si=1.5-3, or may have an atomic ratio of O/Si=2-2.5. Thecoating may have a content of carbon of 20-60 atomic % or 30-40 atomic%. The heat exchanger may comprise a gasket that is at least partiallycoated with the coating. The first heat transfer plate and the secondheat transfer plate may be made of stainless steel.

Further features, objectives, aspects and advantages of the inventionwill appear from the following detailed description as well as from thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example,with reference to the accompanying schematic drawings, in which

FIG. 1 is an exploded view of a block-type heat exchanger with a stackof heat transfer plates,

FIG. 2 is a top view of pairs of heat transfer plates that are used forthe stack of heat transfer plates of FIG. 1.

FIG. 3 is a cross-sectional view along section A-A of FIG. 2.

FIG. 4 is a cross-sectional view along section B-B of FIG. 2.

FIG. 5 is an enlarged view of section C of FIG. 3, and

FIG. 6 is a schematic, cross-sectional view of a coated heat transferplate that is part of the stack of heat transfer plates of FIG. 1.

DETAILED DESCRIPTION

With reference to FIG. 1 a plate heat exchanger 2 of a block-type isshown. The plate heat exchanger 2 comprises a top head 15, a bottom head16 and four side panels 11, 12, 13, 14 that are bolted together with aset of (typically four) corner girders 21-24 for assembling the plateheat exchanger 2. When assembled, the plate heat exchanger 2 has abox-like or block-like shape and an enclosure is formed by the top head15, the bottom head 16 and the side panels 11-14. A stack of heattransfer plates 30 is arranged within the enclosure and comprises, aswill be described in further detail, a number of pairs of heat transferplates. The stack of heat transfer plates 30 also has a box-like orblock-like shape, which shape corresponds to the shape of the enclosureformed by the heads 15, 16 and the side panels 1114. The stack of heattransfer plates 30 has at its corners four linings 31-34 that arearranged to face the corner girders 21-24.

The assembly of the plate heat exchanger 2 is typically performed byusing conventional methods and bolts (not shown) that attach thementioned components to each other via bolt holes like holes 35 and 36.In brief, assembling the plate heat exchanger 2 includes arranging thestack of heat transfer plates 30 on the bottom head 16, sliding thecorner girders 21-24 into the linings 31-34 and bolting them to thebottom head 16. A channel end plate 38 is arranged on top of the stackof heat transfer plates 30 and the top head 15 is bolted to the cornergirders 21-24. Thereafter the side panels 11-14 are bolted to the cornergirders 21-24 and to the heads 15, 16. Generally, the plate heatexchanger 2 also has a base 17 that facilitates attachment of the plateheat exchanger 2 to the ground.

Gaskets, such e.g. gasket 131, are arranged on the side panels 11-14 atsections that face the corner girders 21-24 and the heads 15, 16, suchthat the enclosure formed by the heads 15, 16 and side panels 11-14 isproperly sealed for preventing leakage from the plate heat exchanger 2.

A first side panel 11 and a second side panel 12 of the side panels11-14 comprise inlets and outlets for two fluids. In detail, the firstside panel 11 has an inlet 41 and an outlet 42 for a first fluid. Theinlet 41 and outlet 42 of the first panel 11 form a flow path for thefirst fluid in combination with the stack of heat transfer plates 30,where the flow path extends from the inlet 41, within the stack of heattransfer plates 30 and to the outlet 42. This flow path is illustratedby the broken arrows that extend in directions parallel to the directionD1. Conventional baffles, such as baffle 39, are connected to sides ofthe stack of heat transfer plates 30 for directing the flow of the firstfluid in a number of passes within the stack 30 (four passes in theillustrated figure).

The second side panel 12 has an inlet 43 and an outlet 44 for a secondfluid. The inlet 43 and outlet 44 of the second side panel 12 form aflow path for the second fluid in combination with the stack of heattransfer plates 30, where the flow path extends from the inlet 43,within the stack of heat transfer plates 30 and to the outlet 44. Thisflow path is illustrated by the broken arrows that extend in directionsparallel to the direction D2. Conventional baffles connected to sides ofthe stack of heat transfer plates 30 direct the flow of the second fluidin a number of passes within the stack 30 (here the same number ofpasses as for the first fluid).

The arrangement of baffles is per se accomplished by employingconventional techniques. However, the first flow path for the firstfluid is between the pairs of heat transfer plates in the stack 30,while the second flow path for the second fluid is within the pairs ofheat transfer plates in the stack 30. A pair of heat transfer platescomprises a first heat transfer plate and a second heat transfer plate,as will be described further on. This means that the flow of the firstfluid is between heat transfer plates of different pairs of heattransfer plates, while the flow of the second fluid is between a firstand a second heat transfer plate of the same pair, i.e. within a pair.The linings 31-34 seal the corners of the stack of heat transfer plates30, which ensures that the two different fluids paths are separated.

With reference to FIGS. 2, 3 and 4 a first and a second pair 50, 60 ofheat transfer plates are exemplified, where FIG. 3 is a cross-sectionalview along section A-A of FIG. 2 and FIG. 4 is a cross-sectional viewalong section B-B of FIG. 2. The pairs 50, 60 of heat transfer platesare part of the stack of heat transfer plates 30 illustrated in FIG. 1.The stack 30 comprises a number of pairs of heat transfer plates thatare similar to the pairs 50, 60, such, as 4-200 pairs or even more.

For the pairs 50, 60 of heat transfer plates exemplified by FIGS. 2, 3and 4, the first pair 50 of heat transfer plates comprises a first heattransfer plate 51 and a second heat transfer plate 52. The second pair60 of heat transfer plates is typically similar to the first pair 50 ofheat transfer plates, which means that it also comprises a first heattransfer plate 61 and a second heat transfer plate 62. Thus, the firstheat transfer plate 61 of the second pair 60 of heat transfer plates istypically similar to the first heat transfer plate 51 of the first pair50 of heat transfer plates, while the second heat transfer plate 62 ofthe second pair 60 of heat transfer plates may be similar to the secondheat transfer plate 52 of the first pair 50 of heat transfer plates.

Also, the first heat transfer plate 51 and the second heat transferplate 52 of the first pair 50 of heat transfer plates have similarshapes.

Each heat transfer plate has, as exemplified by the first heat transferplate 51 of the first pair 50 of heat transfer plates, a rectangularshape with a first 511, a second 512, a third 513 and a fourth elongatedside 514. When the stack of heat transfer plates 30 is arranged withinthe enclosure of the plate heat exchanger 2, the first elongated side511 is facing the first side panel 11 while the third side 513 is facingthe third side panel 13. The first heat transfer plate 51 is joined withthe second heat transfer plate 52 via a joint 78 at the first elongatedside 511 and via a joint 79 at the third elongated side 513, as may beseen in FIG. 3.

The first heat transfer plate 51 comprises sets of corrugations 101-106that are arranged on respective sides of elongated joints 72-76 thatjoin the first and second heat transfer plates 51, 52. It may also besaid that the corrugations 101-106 are separated by the elongated joints72-76. The sets of corrugations 101-106 extend a direction that isparallel to the joints 72-76, which direction in the exemplifiedembodiment is parallel to the direction D2. The sets of corrugations101-106 have two outermost sets of corrugations 101, 106, and furtherjoints 71, 77 may be arranged intermediate the outer sets ofcorrugations 101, 106 and the corresponding, closest elongated side 513,511. As previously indicated, since all heat transfer plates may besimilar, all or some of the heat transfer plates of the stack of heattransfer plates 30, such as plates 52, 61 and 62, may have the sameproperties and structural shape as plate 51.

The corrugations 101-106 comprise ridges and grooves that extend in adirection D1 that is 45°-90° transverse a direction D2 along which theelongated joints 71-77 extend. The directions D1. D2 are here the samedirections as previously discussed in respect of the flow of the firstand second fluid. Corrugations 101, 102 on the first heat transfer plate51 and corresponding corrugations 201, 202 on the second heat transferplate 52 each comprise ridges and grooves, such as ridge 92 and groove93 of the first heat transfer plate 51 and ridge 192 and groove 193 ofthe second heat transfer plate 52.

The first pair 50 of heat transfer plates comprises elongated jointgrooves, as exemplified by joint grooves 81-87 of the first heattransfer plate 51, along which the elongated joints 71-77 are arranged.Each corrugation of the set of corrugations 101-106 comprising ridgesand grooves that extend in a direction D1 that is transverse a directionD2 along which the elongated joint grooves 81-87 extend.

The ridges of the first heat transfer plate 51 may be aligned with theridges of the second heat transfer plate 52, as seen in a directionparallel to a normal direction N of the first pair 50 of heat transferplates. This is advantageous in that efficient heat transfer and flow offluid may be accomplished.

As shown, the joints 71-77 are arranged in a respective joint groove81-87. Since the second heat transfer plate 52 is similar to the firstheat transfer plate 51 it also comprises elongated joint grooves alongwhich the elongated joints 71-77 are arranged.

With reference to FIG. 3 and to FIG. 5 illustrating the enlarged sectionC of FIG. 3, it is shown that e.g. joint groove 82 of the first heattransfer plate 51 abut a corresponding joint groove 182 of the secondheat transfer plate 52. The heat transfer plates 51, 52 are then joinedat the joint grooves 82, 182 by virtue of the joint 72. In this context,a backside surface 515 of the joint groove 82 of the first heat transferplate 51 is in contact with a backside surface 525 of the joint groove182 of the second heat transfer plate 52.

The joints are typically formed by welding but may also be formed bybrazing or by some other, suitable means of joining. The heat transferplates 51, 52, 61, 62 are typically made of metal, such as stainlesssteel. When welding is used for forming the joints, i.e. when the jointare welds, laser welding may be used as well as other weldingtechniques, such as resistance welding.

Each of the joints 71-77 may comprise two at least partially overlappingjoint sections, as exemplified by a first section 721 and a secondsection 722 of the joint 72. The joint sections 721, 722 may beoverlapping by a predetermined distance, such as 5-30 mm. The two jointsections 721, 722, or welding sections when the joints are formed bywelding, may begin at a respective end section of the joint groove, asillustrated by the two end sections 821, 822 of joint groove 82.

As indicated, the joining of the first heat transfer plate 51 with thesecond heat transfer plate 52 at the first and third elongated sides511, 513 may be accomplished by a first set of opposite, elongated sidejoints 78, 79, such that a flow path 57 for the second fluid is formedbetween the first set of opposite, elongated side joints 78, 79, i.e.within the first pair 50 of heat transfer plates. The flow path 57 isthen parallel to the direction D2 discussed in connection with FIG. 1.

For facilitating joining of the plates in a pair 50, the first andsecond heat transfer plates 51, 52 have peripheral sections likesections 53, 54 that are folded towards each other. The peripheralsections 53, 54 are folded towards each other since the second heattransfer plate 52 is arranged as an inverted mirror-image of the firstheat transfer plate 51, having in mind that the plates 51, 52 aresimilar. The related weld 79 is applied at a contact surface formedbetween the folded sections 53, 54.

The joint grooves 81-87 may extend unbroken along the flow path 57 thatis formed between the first and second heat transfer plates 51, 52. Alsosince the first heat transfer plate 51 and the second heat transferplate 52 are typically joined by multiple elongated joints 71-77, theflow path 57 for the second fluid formed between the first and secondheat transfer plates 51, 52 comprises multiple parallel flow channels571-576.

To form the stack of heat transfer plates 30, pairs of heat transferplates like the first pair 50 of heat transfer plates and the secondpair 60 of heat transfer plates are joined via opposite, elongated sidejoints. Such joints are exemplified by a set of opposite, elongated sidejoints 781, 782 arranged between the first pair 50 of heat transferplates and the second pair 60 of heat transfer plates. Such elongatedside joints 781, 782 are transverse the first set of elongated sidejoints 78, 79 and joins a pair of heat transfer plates (exemplified bypair 50) with an adjacent pair of heat transfer plates (exemplified bypair 60). For facilitating joining, the plates 51, 52, 61, 62 haverespective peripheral sections that are folded towards a heat transferplate that belongs to another pair of heat transfer plates, such asfolded sections 56 and 65. The related weld 781 is applied at a contactsurface formed between the folded sections 56, 65.

When the pairs 50, 60 of heat transfer plates are joined, a flow path 67for the first fluid is formed between the pairs 50, 60 of heat transferplates. Since the pairs 50, 60 are joined only at the second set of sidejoints 781, 782 a so called free-flow path is formed between the joints781, 782, i.e. a free-flow path is formed between the pairs 50, 60 ofheat transfer plates. A free-flow path may in this context be defined asa flow path without any contact points intermediate the side joints 781,782. Generally, free-flow has been observed to be advantageous sinceoccurrence of e.g. deposits from the fluid or the presence of bacteriamay be reduced or, in practice, even eliminated.

To form the complete stack of heat transfer plates 30, a number of pairsof heat transfer plates are stacked adjacent each other and joined toeach other in a manner like the joining of the first and the secondpairs 50, 60 of heat transfer plates. The joining of the pairs may beaccomplished by using the same methods (welding, brazing etc.) as whenjoining the plates of one pair.

For efficiently joining the heat transfer plates to the linings 31-34each heat transfer plate has four protrusions at its corners, such asprotrusions 515-518 of the first heat transfer plate 51. The protrusionsare then joined to the linings 31-34 by e.g. welding, brazing or by someother suitable means of joining. The linings 31-34 partially surroundthe set of corner girders 21-24 when the plate heat exchanger 2 isassembled, such that the stack of heat transfer plates 30 is firmlyfixed within the enclosure that is formed by the heads 15, 16 and theside panels 11-14.

The heat transfer plates 51, 52, 61, 62 may per se be manufactured fromsteel sheets that are pressed with a press tool that forms thecorrugations and the weld grooves. A cutting machine thereafter cuts thepressed plates along their periphery and the edges of the cut plates arefolded in a machine that forms the folded, peripheral sections.

The heat transfer plates in the stack of heat transfer plates 30comprises a coating. The coating may be referred to as a non-stickcoating and makes it easy to clean the plates. The coated plates provideimproved heat transfer over time compared to conventional heat transferplates since the latter gets fouled much quicker, which decreases theheat transfer performance to a larger extent. The coating also resultsin a much more even surface on the plates, which gives better flowcharacteristics. Also, a pressure drop over the plates is reduced overtime for the plate heat exchanger 2 in comparison with conventionalblock-type plate heat exchangers, since the buildup of impurities,microorganisms and other substances is reduced.

The coated plates may easily be cleaned by using high pressure washingwith water. Moreover, there is no need for extensive, time consumingmechanical cleaning or cleaning using strong acids, bases or detergents,such as e.g. NaOH and HNO₃.

The heat transfer plates in the stack 30 are in a sol-gel process coatedwith a coating that comprises organosilicon compounds. The organosiliconcompounds are starting materials that are used in the sol-gel processand are preferably silicon alkoxy compounds. In the sol-gel process asol is converted into a gel to produce nano-materials. Throughhydrolysis and condensation reactions a three-dimensional network ofinterlayered molecules is produced in a liquid. Thermal processingstages are then used to process the gel further into nano-materials ornanostructures, which results in a final coating. The coating comprisingsaid nano-materials or nanostructures mainly comprise silicon oxide,SiO_(x), having an atomic ratio of O/Si>1, alternatively an atomic ratiowithin the range of O/Si=1.5-3, or alternatively within the range ofO/Si=2-2.5. By an “atomic ratio of O/Si>1” is meant that the number ofOxygen atoms (O) of the silicon oxide (SiO_(x)) divided by the number ofSilicon atoms (Si) of the silicon oxide (SiO_(x)) is larger than one.Correspondingly, for the alternatives the number of Oxygen atoms (O)divided by the number of Silicon atoms is within the range of 1.5-3, orwithin the range of 2-2.5.

A preferred silicon oxide is silica, SiO₂. The siliconoxide forms athree dimensional network having excellent adhesion to the plates. Allheat transfer plates of the stack 30, such as the first heat transferplate 51 and the second heat transfer plate 52, may be coated. Typicallythe plates are coated on the sides that face either one or both of theflow path for the first fluid and the flow path for the second fluid.

The coating has a content of carbon originating from hydrocarbon chains.The hydrocarbons chains may have functional groups such as those foundin hydrocarbon chains or aromatic groups, e g C═O, C—O, C—O—C, C—N,N-C-O, N-C═O, etc. Preferably the content of the carbon is ≧10 atomic %,or in the range of 20-60 atomic %, or in the range of 30-40 atomic %.The carbon impart flexibility and resilience to the coating which isimportant if the plates during operation flex due to high pressuresexerted on the plates in the stack 30. The hydrocarbon chains arehydrophobic and oleophobic, which results in the non-stick properties ofthe coating.

With reference to FIG. 6 a schematic view is shown where the first heattransfer plate 51 is provided with a siliconoxide sol gel coating 701 asdescribed above. The coating is also referred to as siliconoxide layer701. Closest to the plate 51 the siliconoxide layer 701 forms aninterface 702 between the coating siloxane and a metal oxide film of theplate 51. A bulk of the coating 701 is the siloxane network 703 that hasorganic linker chains and voids that impart flexibility to the coating701. The siloxane network 703 is “on top” of the interface 702. Thesiliconoxide layer 701 forms an outermost layer in from of a functionalsurface 704 that has hydrophobic and oleophobic properties that reducefouling. There are no sharp boundaries between the interface 702 and thesiloxane network 703 respectively the siloxane network 703 and thefunctional surface 704, but rather gradual transitions

All plates in the stack 30 that are coated may have the coatingdescribed in connection with FIG. 6. The coating is both durable andflexible and provides a plate for a block-type plate heat exchanger thathas excellent non-stick properties and wear- and crack-resistance.

In one embodiment at least one sol comprising organosilicon compounds isapplied to the surface of the heat transfer plates that are coated. Thesurface may be wetted/coated with the sol in any suitable way. Thesurface coating may e.g. be applied by spraying, dipping or flooding.Typically, all surfaces of a heat transfer plate that is in contact witha fluid that may cause fouling are coated. Also, the gaskets like gasket131 arranged on the side panels 11-14 may be coated, typically with thesame type of coating that is used for the heat transfer plates. Thecoating is then typically applied at least on the surfaces of thegaskets that are in contact with the fluid that may cause fouling.

A method of coating the heat transfer plates of the stack 30 comprisespretreatment of at least the surfaces on the heat transfer plates to becoated. This pretreatment may be carried out by means of dipping,flooding or spraying. The pretreatment is used to clean the surfaces tobe coated in order to obtain increased adhesion of the coating. Examplesof pretreatments are treatment with acetone and/or alkaline solutions,e.g. caustic solution.

The method of coating the heat transfer plates may comprise thermalprocessing stages, e.g. a drying operation may be carried out after apretreatment and a drying and/or curing operation may be used after thecoating of the plate has taken place. The coating may be subjected toheat by using conventional heating apparatuses, such as ovens.

The coating, which as indicated comprises SiOx, is applied to the platesof the stack 30. The application of the coating is done by means ofsol-gel processing. The coating is preferably between 1 and 30 μm thick.A coating thickness below 1 μm is considered being not enough wearresistant since the plates in the plate heat exchanger 2 are able toflex slightly during operation. Flexing of the plates causes wear on thecoating and with time the coating wear down. Still, the thickness of thecoating has an upper limit since the application of substances on theheat transfer plates influences the their heat transfer capability andthus the overall performance of the plate heat exchanger. The upperlimit for the thickness of the coating is preferably 30 μm. Thus, thecoating thickness of the silicon oxide sol containing coating is 1-30μm, and in alternatives preferably 1.5-25 μm, preferably 2-20 μm,preferably 2-15 μm, preferably 2-10 μm or preferably 3-10 μm.

The material of which the heat transfer plates in the stack 30 are madeof may be chosen from several metals and metal alloys. Preferably, thematerial is stainless steel or titanium. The material may also be chosenfrom nickel, copper, any alloys of the mentioned metals and/or carbonsteel.

In an attempt to find more a foul resistant block-type plate heatexchanger, tests were conducted on two low surface energy glass ceramiccoatings of which both are of the type of coating described above. Thetested coatings are referred to as Coat 1 and Coat 2. The tests, theanalysis and the results are presented below. Coat 1 is a silanterminated polymer in butyl acetate and Coat 2 is a polysiloxan-urethanresin in solvent naphtha/butylacetate. The test were performed on coatedheat transfer plates in the stack 30. In the following a plate for whichtests is performed is also referred to as “substrate”.

The tests shows properties of the coatings in respect of substratewetting, substrate adhesion, contact angle, coating thickness andstability against 1.2% HNO₃ in H₂O, 1% NaOH in H₂O and crude oil. Theresults are summarized below in Table 1.

TABLE 1 Coat 1 Coat 2 Substrate Excellent Excellent wetting SubstrateAl: 0/0 Al: 0/0 adhesion Stainless steel: 0/0 Stainless steel: 0/0 Ti:0/0 (see below) Ti: 0/0 (see below) Contact angle H2O: 102-103° H2O:102-103° measurements Coating 4-10 μm 2-4 μm thickness Stability 1.2%HNO3 in H2O: 1.2% HNO3 in H2O: 1½ h at 75° C. 1½ h at 75° C. 1% NaOH inH2O: 3 h at 1% NaOH in H2O: 2 h at 85° C. 85° C. Crude oil: 6 monthsCrude oil: 6 months at 20° C. at 20° C.

Both coatings showed excellent wetting when spray coated onto eitherstainless steel or titanium substrates.

Adhesion was determined by cross-cut/tape test according to the standardDIN EN ISO 2409. Rating is from 0 (excellent) to 5 (terrible). 0 or 1 isacceptable while 2 to 5 is not. First digit indicates rating after crosscut (1 mm grid) and the second digit gives rating after tape has beenapplied and taken off again.

To obtain proper adhesion for Coat 1 and Coat 2 the substrates weresubjected to pre-treatment. To obtain a proper adhesion of Coat 1 onstainless steel the substrate was pre-treated by submerging it in analkaline cleaning detergent for 30 minutes. Next the substrate waswashed with water and demineralized water and dried before Coat 1 wasapplied (applied within half an hour to achieve optimal adhesion). Testshave shown that the adhesion is reduced if cleaning of the substrate isonly carried out with acetone. Pre-treatment was also used for stainlesssteel substrates that are coated with Coat 2. This coating displayedunaffected adhesion whether an alkaline detergent or acetone was used aspre-treatment or not. If the pre-treatment step is neglected or notproperly made the coating adhesion will be effected.

Both coatings showed good stability under acidic condition. The coatingswere stable for 1½% hours at 75° C. and more than 24 hours at roomtemperature.

Under alkaline conditions Coat 1 showed a better result than Coat 2.Coat 1 could withstand the alkaline conditions for 3 hours at 85° C. andCoat 2 for 2 hours at 85° C. Both coatings showed no decomposition orreduction in oleophobic properties after being subjected to crude oilfor 6 months at a temperature of 20° C.

Heat transfer plates in the stack 30 were then coated with Coat 1 andCoat 2. The heat exchanger plates were in this test made of titanium andthe heat exchanger 2 was used in a crude oil application. All coatedheat transfer plates underwent pre-treatment, which comprised treatmentwith acidic and alkaline solutions to remove fouling and high pressurewashing of the plates with water. The plates were left to dry beforeapplication of coating.

The pre-treatment was completed a day before Coat 1 and Coat 2 wereapplied to the plates. As the plates have been left to dry at ambienttemperature (approximately cover 20° C.), some plates were still wet.More precisely, a third of the plates were coated with Coat 1 and athird of the plates were coated with Coat 2, while a remaining third ofthe plates were kept uncoated. The coating is accomplished by sprayingthe respective coat into the flow paths 57, 67 that are formed by theplats in the stack 30, such that the sides of the that faces the flowpaths are coated. The thickness of the coating was measured to be 2-4μm. Curing/drying for the two coatings was performed for 1½ hours in anoven at elevated temperatures of 200° C. respectively 160° C.

The stack 30 with the coated heat transfer plates were then arranged inthe heat exchanger of FIG. 1 and an evaluation of the coated plates wasperformed after about seven months of operation of the plate heatexchanger 2.

The plates were analyzed after the seven months. In detail, threedifferent silicon oxide-coated heat transfer plates were analyzed bymeans of XPS (X-ray Photoelectron Spectroscopy), also known as ESCA(Electron Spectroscopy for Chemical Analysis). The XPS method providesquantitative chemical information, including a chemical compositionexpressed in atomic % for the outermost 2-10 nm of a surface.

A measuring principle of the XPS method comprises that a sample (i.e. aheat transfer plate coated with Coat 1, a heat transfer plate coatedwith Coat 2 and an uncoated plate) is placed in high vacuum and isirradiated with well defined x-ray energy, which results in an emissionof photoelectrons from the sample. Only photoelectrons from theoutermost surface of the sample reach the detector. By analyzing thekinetic energy of the photoelectrons, their binding energy can becalculated, thus giving their origin in relation to a chemical element(including the electron shell) of the sample.

XPS provided quantitative data on both the elemental composition anddifferent chemical states of a chemical element of the sample (such asdifferent functional groups, chemical bonding, oxidation state, etc).All chemical elements except hydrogen and helium are detected and theobtained chemical composition of the sample is expressed in atomic %.

XPS spectra were recorded using a Kratos AXIS Ultra^(DLD) x-rayphotoelectron spectrometer. The samples were analyzed using amonochromatic Al x-ray source. The analysis area was below 1 mm². In theanalysis so a called wide spectra run was performed to detect chemicalelements present in the surface of the sample. The relative surfacecompositions were obtained from quantification of each chemical element.

When heat transfer plates with different types (in respect of a contentof C, O and Si) of the silicon oxide coating described herein areanalyzed, or more precisely when the chemical elements of the coating isanalyzed, a relative surface composition in atomic % and an atomic ratioO/Si may be found. It has then been observed that mainly C, O and Si maybe detected on the outermost surfaces of the coating. A content of C istypically 41.9-68.0 atomic %, a content of O is 19.5-34.3 atomic % whilea content of Si is 8.6-23.4 atomic %. The atomic ratio O/Si is1.46-2.30. Note that for the atomic ratio O/Si, the total amount ofoxygen is used. This means that also oxygen in functional groups withcarbon is included. Otherwise, for silica a theoretical ratio O/Si of2.0 is expected (i.e. SiOx in form of SiO₂).

After four months of operation a pre-inspection by thermo-imaging wasperformed. A thermo-image was taken of a mid region of the heatexchanger 2 when the heat exchanger was operated. From the image it wasobvious that some heat transfer plates show increased heat transfercompared to other heat transfer plates in the heat exchanger.

The inspection showed an elevated temperature at the coated plates. Thenon-coated plates showed a lower operating temperature. The differencein temperature is an effect of different fouling, where coated plats haselevated temperatures.

A visual inspection revealed that the plates with the coating designatedCoat 1 was covered with the least amount of fouling on the crude oilfacing plate side. Also, Coat 2 had a reduced amount of fouling on thecrude oil facing plate side compared to the bare titanium surface, butto a lesser extent then Coat 1. The bare titanium plates were completelycovered in a thick layer of crude oil that “fouled” the plates. The term“fouling” is here used to describe deposits formed on the heat transferplates during operation. The fouling is residues and deposits formed bythe crude oil and consists of a waxy, organic part and amineral/inorganic part.

By subtracting the average weight of a clean plate from the weightrecorded for the individual fouled plates the average amount of foulingper surface type was calculated (table 2). The weight of the coating wasnot compensated for and so the real fouling reduction is slightlyhigher. For the heat transfer plates used in the test the heat transfersurface is 0.85 m², so for a plate with a 4 μm thick coating on bothsides the total volume of coating material is around 6.8 cm³. If thecoating is estimated to be pure SiO₂ (density 2.6 g/cm³) then the amountof coating per plate is about 20 g.

TABLE 2 Average Fouling Surface fouling (g) reduction (%) Titanium 585 —Coat 1 203 65 Coat 2 427 27

For both Coat 1 and Coat 2 the fouling of the plates were more easilyremoved compared to the fouling on bare titanium plates, see Table 3.The difference in cleaning requirements was tested by manually wiping ofthe plates with a tissue and by high pressure water cleaning. Justwiping the plates with a tissue showed that the fouling was very easilyremoved from the coated plates, contrary to the uncoated plates. Byusing high pressure water cleaning all fouling except for one or twosmall patches could be removed from the Coat 1 coated surface. On theCoat 2 coated surface somewhat more fouling was present after water jetcleaning. This fouling had the form of slightly burnt oil. The coatingwas in a good condition. The crude oil has passed through the first flowpath of the heat exchanger 2, while sea water has passed through thesecond flow path. On plate surfaces that face the seawater both coatingshad deteriorated.

TABLE 3 Coat 1 Coat 2 Uncoated View very little fouling reduced foulingfouling significant and widespread Wipe very easy to very easy tofouling was not with remove fouling remove fouling removed tissue Highthe plates most of the fouling even after attempts pressure appeared asnew was removed of manual removal water of fouling, still a washingconsiderable layer remains

The coatings resistance to cold conditions was tested submerging theplates in liquid nitrogen having a temperature of −196° C. Next theplates were washed by high pressure water, which removed almost allfouling. No coating failure was observed for either Coat 1 or Coat 2.

Turning back to FIGS. 1, 2 and 4, the plate heat exchanger 2 haspredetermined measurements for a number of the components it comprises.For example, the first heat transfer plate and the second heat transferplate may have a thickness m1 of 0.6-1.4 mm or 0.8-1.2. Each of thefirst heat transfer plate and the second heat transfer plate may have aheat transfer area m2 of 0.05-0.30 m² or 0.6-1.8 m². Any of the top headand the bottom head may have a thickness m3 of 45-145 mm or 190-250 mm.Each of the four side panels may have a thickness m4 of 35-85 mm or65-175 mm. Each of the corner girders may comprise a cross-sectionalside m5 that measures 35-85 mm or 110-190 mm. Finally, the sealedenclosure may have a volume of maximum 0.02-0.40 m³ or 0.7-5.0 m³. Asexplained, these measurements provides, each alone or in combination,conditions where the heat transfer plates in the stack 30 flex lesswhich allows the coating to remain on the heat transfer plates for alonger period of time. Still, the components are not unnecessarilyover-dimensioned but the measurements has been optimized in respect ofallowing the coating to remain for a longer period of time while stillassuring that reasonable amounts of materials are used for the heatexchanger 2.

In detail, the measurements m1-m5 may be optimized in respect of eachother. For example, in one embodiment the first heat transfer plate andthe second heat transfer plate have a thickness of 0.7-0.9 mm and a heattransfer area of 0.02-0.035 m², while any of the top head and the bottomhead has a thickness of 35-45 mm, each of the four side panels may has athickness of 35-45 mm, each of the corner girders comprises across-sectional side that measures 35-45 mm, and the sealed enclosurehas a volume of 0.005-0.020 m³.

In another embodiment the first heat transfer plate and the second heattransfer plate have a thickness of 0.7-0.9 mm and a heat transfer areaof 0.05-0.07 m², while any of the top head and the bottom head has athickness of 45-55 mm, each of the four side panels may has a thicknessof 35-65 mm, each of the corner girders comprises a cross-sectional sidethat measures 45-55 mm, and the sealed enclosure has a volume of0.02-0.06 m³.

In another embodiment the first heat transfer plate and the second heattransfer plate have a thickness of 0.7-0.9 mm and a heat transfer areaof 0.09-0.11 m², while any of the top head and the bottom head has athickness of 45-55 mm, each of the four side panels may has a thicknessof 35-65 mm, each of the corner girders comprises a cross-sectional sidethat measures 45-55 mm, and the sealed enclosure has a volume of0.04-0.22 m³.

In another embodiment the first heat transfer plate and the second heattransfer plate have a thickness of 0.9-1.1 mm and a heat transfer areaof 0.13-0.19 m², while any of the top head and the bottom head has athickness of 60-80 mm, each of the four side panels may has a thicknessof 45-85 mm, each of the corner girders comprises a cross-sectional sidethat measures 55-65 mm, and the sealed enclosure has a volume of0.12-0.26 m³.

In another embodiment the first heat transfer plate and the second heattransfer plate have a thickness of 0.9-1.1 mm and a heat transfer areaof 0.24-0.30 m², while any of the top head and the bottom head has athickness of 120-160 mm, each of the four side panels may has athickness of 45-85 mm, each of the corner girders comprises across-sectional side that measures 65-105 mm, and the sealed enclosurehas a volume of 0.2-0.6 m³.

In another embodiment the first heat transfer plate and the second heattransfer plate have a thickness of 0.9-1.1 mm and a heat transfer areaof 0.50-0.80 m², while any of the top head and the bottom head has athickness of 170-230 mm, each of the four side panels may has athickness of 90-160 mm, each of the corner girders comprises across-sectional side that measures 100-140 mm, and the sealed enclosurehas a volume of 1.0-2.4 m³.

In another embodiment the first heat transfer plate and the second heattransfer plate have a thickness of 1.1-1.3 mm and a heat transfer areaof 1.4-2.0 m², while any of the top head and the bottom head has athickness of 120-400 mm, each of the four side panels may has athickness of 110-250 mm, each of the corner girders comprises across-sectional side that measures 120-240 mm, and the sealed enclosurehas a volume of 2.4-5.9 m³.

From the description above follows that, although various embodiments ofthe invention have been described and shown, the invention is notrestricted thereto, but may also be embodied in other ways within thescope of the subject-matter defined in the following claims. Forexample, optimization calculations may show that other measurements forcomponents of the heat exchanger may provide a structure that allows thecoating to remain on the coated surface for long period of time. Also,the heat transfer plates may have another pattern of corrugation thanthe shown one. In other embodiments the elongated joints and theirassociated joint grooves on the heat transfer plates may be omitted suchthat e.g. corrugations cover the heat transfer areas of the plates.

1. A plate heat exchanger comprising: a top head, a bottom head and fourside panels that are bolted together with a set of corner girders toform a sealed enclosure, and a stack of heat transfer plates that isarranged in the sealed enclosure, the stack of heat transfer platescomprising: pairs of heat transfer plates that are stacked such that aflow path for a first fluid is formed between the stacked pairs of heattransfer plates, wherein a pair of the stacked pairs of heat transferplates comprises a first heat transfer plate and a second heat transferplate that are joined such that a flow path for a second fluid is formedbetween the first and second heat transfer plates, the first heattransfer plate and the second heat transfer plate comprising a coatingthat has a layer thickness of 1-30 μm; is prepared by sol-gelprocessing; comprises silicon oxide (SiOx) having an atomic ratio ofO/Si>1; and comprises ≧5 atomic percent carbon (C).
 2. A heat exchangeraccording to claim 1, wherein the first heat transfer plate and thesecond heat transfer plate has a thickness (m1) of 0.6-1.4 mm.
 3. A heatexchanger according to claim 1, wherein each of the first heat transferplate and the second heat transfer plate has a heat transfer area (m2)of 0.05-0.30 m².
 4. A heat exchanger according to claim 1, wherein anyof the top head and the bottom head has a thickness (m3) of 45-145 mm.5. A heat exchanger according to claim 1, wherein each of the four sidepanels has a thickness (m4) of 35-85 mm.
 6. A heat exchanger accordingto claim 1, wherein each of the corner girders comprises across-sectional side (m5) that measures 35-85 mm.
 7. A heat exchangeraccording to claim 1, wherein the sealed enclosure has a volume of0.02-0.40 m³.
 8. A heat exchanger according to claim 1, wherein thelayer thickness of the coating is 1.5-25 μm.
 9. A heat exchangeraccording to claim 1, wherein the silicon oxide, SiOx, has an atomicratio of O/Si=1.5-3.
 10. A heat exchanger according to claim 1, whereinthe coating has a content of carbon of 20-60 atomic %.
 11. A heatexchanger according to claim 1, comprising a gasket that is at leastpartially coated with the coating.
 12. A heat exchanger according toclaim 1, wherein the first heat transfer plate and the second heattransfer plate are made of stainless steel.
 13. A heat exchangeraccording to claim 1, wherein the layer thickness of the coating is 2-20μm.
 14. A heat exchanger according to claim 1, wherein each of the firstheat transfer plate and the second heat transfer plate has a heattransfer area (m2) of 0.6-1.8 m².
 15. A heat exchanger according toclaim 1, wherein any of the top head and the bottom head has a thickness(m3) of 190-250 mm.
 16. A heat exchanger according to claim 1, whereinthe layer thickness of the coating is 3-10 μm.
 17. A heat exchangeraccording to claim 1, wherein the silicon oxide, SiOx, has an atomicratio of O/Si=2-2.5.
 18. A heat exchanger according to claim 1, whereinthe coating has a content of carbon of 30-40 atomic %.
 19. A heatexchanger according to claim 1, wherein the first heat transfer plateand the second heat transfer plate has a thickness (m1) of 0.8-1.2 mm.20. A heat exchanger according to claim 1, wherein each of the four sidepanels has a thickness (m4) of 65-175 mm.